Two main questions on venture investors’ minds: “How do we know if accelerators are succeeding?” And: “Are we in an accelerator bubble?”

The managing director of Foundry Group and TechStars co-founder Brad Feld said he thinks there is no “accelerator bubble.” In fact, he wants to see an accelerator established in every town with a population of at least 100,000 in the U.S., to foster a healthy, local and national economy.

He also urged investors to be patient in their judgment of these programs.

The first “true accelerator,” Y Combinator, started in 2005, he noted, and TechStars followed shortly after. By 2008, many more accelerators began cropping up, like DreamIt–one of the first such programs on the East Coast. But it took until 2009 for even the earliest players to enroll a “meaningful number of companies,” Feld observed.

For its part, TechStars measures and publicly reports data including how many companies apply and go through its various programs. After they leave, it notes how many startups remain in business, how much follow-on funding they raise from outside investors, and whether they have exited or shut down. “Funding in and of itself is not a measure of success,” Feld cautioned.

About one in 10 of TechStars companies “bootstraps,” and doesn’t raise funding from outside investors, but still creates jobs and wealth, he said. Early classes from TechStars Boulder have still-active companies delivering 2x and 3x returns to investors, so far.

Springboard Enterprises–which runs accelerators focused on health-care and media ventures led by women–also tries to measure serial entrepreneurship, and alumni networking by its participants.

A director of programming at Springboard Enterprises, Joshua Henderson, explained, “We want to see founders who support a next crop of companies as mentors or angel investors [to them]. We also like to see entrepreneurs go on to start new ventures.”

Rock Health founder Halle Tecco said specialized accelerators should have additional metrics tailored to their missions. Her program, which launched in 2010, focuses on digital-health startups.

“Follow-on funding is a good litmus test for whether startups are scalable and sustainable, true,” Tecco said. “But in digital health, we want to see companies making a positive impact to clinical outcomes. We’re figuring out how to measure that, still. Is it pounds lost, or errors reduced at hospitals? We’re learning everything we can.”

Unlike TechStars and Springboard, Rock Health is run as a nonprofit accelerator, with a for-profit seed fund, which offers $100,000 seed-stage investments to each accepted startup.

A pioneer of the venture capital industry, David T. Morgenthaler, said accelerators help venture capital firms by forcing entrepreneurs to “fail fast,” and truly understand their risks before they even consider raising venture capital.

But Morgenthaler agreed with Feld, generally, that there is no accelerator bubble yet, and that it is probably too early for investors to say how well these organizations perform, both as sources of dealflow for venture capitalists and as early-stage funds that generate real returns for limited partners.